The Human Balance System—

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The Human Balance System— A Complex Coordination of Central and Peripheral Systems By the Vestibular Disorders Association, with contributions by Mary Ann Watson, MA, and F. Owen Black, MD, FACS

Good balance is often taken for granted. the eye and body muscles. Injury, Most people don’t find it difficult to walk disease, or the aging process can affect across a gravel driveway, transition from one or more of these components. walking on a sidewalk to grass, or get out of bed in the middle of the night without Sensory input stumbling. However, with impaired Maintaining balance depends on infor- balance such activities can be extremely mation received by the brain from three fatiguing and sometimes dangerous. peripheral sources: eyes, muscles and Symptoms that accompany the joints, and vestibular organs (Figure 1). All unsteadiness can include dizziness, three of these sources send information to vertigo, hearing and vision problems, and the brain in the form of nerve impulses difficulty with concentration and memory. from special nerve endings called sensory receptors. What is balance? Balance is the ability to maintain the Input from the eyes body’s center of mass over its base of Sensory receptors in the retina are called support.1 A properly functioning balance rods and cones. When light strikes the system allows humans to see clearly rods and cones, they send impulses to the while moving, identify orientation with brain that provide visual cues identifying respect to gravity, determine direction how a person is oriented relative to other and speed of movement, and make auto- objects. For example, as a pedestrian matic postural adjustments to maintain walks along a city street, the surrounding posture and stability in various conditions buildings appear vertically aligned, and and activities. each storefront passed first moves into and then beyond the range of peripheral Balance is achieved and maintained by vision. a complex set of sensorimotor control systems that include sensory input from Input from the muscles and joints vision (sight), proprioception (touch), and Proprioceptive information from the skin, the vestibular system (motion, equilibri- muscles, and joints involves sensory um, spatial orientation); integration of receptors that are sensitive to stretch or that sensory input; and motor output to pressure in the surrounding tissues. For © Vestibular Disorders Association ◦ www.vestibular.org ◦ Page 1 of 5

example, increased pressure is felt in the Proprioceptive cues from the neck indicate front part of the soles of the feet when a the direction in which the head is turned. standing person leans forward. With any Cues from the ankles indicate the body’s movement of the legs, arms, and other movement or sway relative to both the body parts, sensory receptors respond by standing surface (floor or ground) and the sending impulses to the brain. quality of that surface (for example, hard, soft, slippery, or uneven). The sensory impulses originating in the neck and ankles are especially important. Input from the vestibular system

Sensory information about motion, particular canal, the endolymphatic fluid equilibrium, and spatial orientation is within it lags behind because of inertia provided by the vestibular apparatus, and exerts pressure against the canal’s which in each ear includes the utricle, sensory receptor. The receptor then sends saccule, and three semicircular canals. impulses to the brain about movement. The utricle and saccule detect gravity When the vestibular organs on both sides (vertical orientation) and linear of the head are functioning properly, they movement. The semicircular canals, which send symmetrical impulses to the brain. detect rotational movement, are located (Impulses originating from the right side at right angles to each other and are filled are consistent with impulses originating with a fluid called endolymph. When the from the left side.) head rotates in the direction sensed by a

Integration of sensory input provided by the vestibular organs may Balance information provided by the help override this sensory conflict. In peripheral sensory organs—eyes, muscles addition, higher level thinking and and joints, and the two sides of the memory might compel the person to vestibular system—is sent to the brain glance away from the moving bus to look stem. There, it is sorted out and inte- down in order to seek visual confirmation grated with learned information that his body is not moving relative to the contributed by the cerebellum (the pavement. coordination center of the brain) and the cerebral cortex (the thinking and memory Motor output center). The cerebellum provides As sensory integration takes place, the information about automatic movements brain stem transmits impulses to the that have been learned through repeated muscles that control movements of the exposure to certain motions. For example, eyes, head and neck, trunk, and legs, by repeatedly practicing serving thus allowing a person to both maintain a ball, a tennis player learns to optimize balance and have clear vision while balance control during that movement. moving. Contributions from the cerebral cortex include previously learned information; Motor output to the muscles and joints for example, because icy sidewalks are A baby learns to balance through practice slippery, one is required to use a different and repetition as impulses sent from the pattern of movement in order to safely sensory receptors to the brain stem and navigate them. then out to the muscles form a new pathway. With repetition, it becomes easier Processing of conflicting for these impulses to travel along that sensory input nerve pathway—a process called A person can become disoriented if the facilitation—and the baby is able to sensory input received from his or her maintain balance during any activity. eyes, muscles and joints, or vestibular Strong evidence exists suggesting that organs sources conflicts with one another. such synaptic reorganization occurs For example, this may occur for example, throughout a person’s lifetime of when a person is standing next to a bus adjusting to changing motion environs. that is pulling away from the curb. The visual image of the large rolling bus may This pathway facilitation is the reason create an illusion for the pedestrian that dancers and athletes practice so he or she—rather than the bus—is arduously. Even very complex movements moving. However, at the same time the become nearly automatic over a period of proprioceptive information from his time. muscles and joints indicates that he is not actually moving. Sensory information For example, when a person is turning

cartwheels in a park, impulses systems. Its interlacing feedback mecha- transmitted from the brain stem inform nisms can be disrupted by damage to one the cerebral cortex that this particular or more components through injury, activity is appropriately accompanied by disease, or the aging process. Impaired the sight of the park whirling in circles. balance can be accompanied by other With more practice, the brain learns to symptoms such as dizziness, vertigo, interpret a whirling visual field as normal vision problems, nausea, fatigue, and during this type of body rotation. concentration difficulties. Alternatively, dancers learn that in order to maintain balance while performing a The complexity of the human balance series of pirouettes, they must keep their system creates challenges in diagnosing eyes fixed on one spot in the distance as and treating the underlying cause of long as possible while rotating their body. imbalance. Vestibular dysfunction as a cause of imbalance offers a particularly Motor output to the eyes intricate challenge because of the The vestibular system sends motor control vestibular system’s interaction with signals via the nervous system to the cognitive functioning,2 and the degree of muscles of the eyes with an automatic influence it has on the control of eye function called the vestibulo-ocular reflex. movements and posture. When the head is not moving, the number of impulses from the vestibular organs on References the right side is equal to the number of 1. Shumway-Cook A, Woollacott MH. impulses coming from the left side. When Motor Control: Theory and Practical the head turns toward the right, the num- Applications. Philadelphia: Lippincott, ber of impulses from the right ear Williams & Wilkins; 2001. increases and the number from the left 2. Hanes DA, McCollum G. Journal of ear decreases. The difference in impulses Vestibular Research 2006;16(3):75–91. sent from each side controls eye movements and stabilizes the gaze during © 2008 Vestibular Disorders Association active head movements (e.g., while VEDA’s publications are protected under running or watching a hockey game) and copyright. For more information, see our passive head movements (e.g., while permissions guide at www.vestibular.org. sitting in a car that is accelerating or This document is not intended as a decelerating). substitute for professional health care.

The coordinated balance system The human balance system involves a complex set of sensorimotor-control

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